In a multi-stage interconnection network using optical interconnection for use within multi-processor computer architecture, it is necessary to have an optical transpose system. Thus, in principle, optical interconnect technologies offer several advantages over electrical systems. Connections can be made at higher speeds with less crosstalk and less power consumption than electrical channels. The power required is almost independent of the length of the connection, at least over the length of connections involved within a parallel configuration.
FIG. 1 illustrates a generic electronic sub-system consisting of N modules in a first stage connected to M modules in a second stage. If a signal path is to exist between each first stage module and each second stage module, each first stage module requires M output ports, each of which is linked to a respective input port of each of the second stage modules. Conversely and equivalently, each second stage module requires N input ports each of which is linked to a respective output port of each first stage module N. In the sub-system illustrated N=4 and M=3, through it will be appreciated that these numbers would be different in different applications.
Let the symbol n index modules in the first stage and the input ports of each second stage module, and the symbol m index modules in the second stage and the output ports of each module in the first stage. The interconnection is then described by the transposition:(n,m)→(m,n)that is to say the port m of the module n in the first stage is connected to the port n of module m in the second stage.
There are advantages in implementing interconnections optically. FIG. 1 graphically illustrates that the transpose interconnection involves a large number of crossovers. This presents problems for a planar technology implementation, but does not present problems for a free-space optics implementation. It is then more natural to group the ports of each module and to group the modules together in two-dimensional arrays substantially in a plane.
An optical transpose system is described in an article in Optics Letters, 18 pages 1083-1085 (1993) by G C Marchand, P Harvey and S C Essener entitled ‘The Optical Transpose Interconnection System Architectures. This optical system has two stages, each consisting of an array of mesolenses that image the light from an array of light sources on an input plane onto an array of receiving devices on an output plane. The optical system suffers from severe aberrations due to the off-axis imaging arrangement of similar-sized mesolens. As described, the mesolens intercept only a fraction of the light emerging from a divergent source such as a light emitting diode (LED), and this leads to a high insertion loss and the potential for crosstalk if the light not captured is not fully blocked. Spatially-coherent sources individually directed into the appropriate mesolens could overcome these problem, but would require beam steering components at the input plane; and also, in the case of a monomode receiving device, at the output plane.